1. Field of the Invention
The present invention relates to a nonvolatile magnetic memory device.
2. Description of Related Art
With the remarkable proliferation of information and communication equipment, particularly, personal small items such as portable terminals, various kinds of semiconductor devices of memory elements and logic elements forming the equipment are requested to have higher performance of higher integration, higher speed, lower power consumption, and the like. Especially, a nonvolatile memory is considered to be indispensable for the ubiquitous age. Even in exhaustion or trouble of a power supply, or disconnection between a server and a network due to some failure, important information can be saved and protected by the nonvolatile memory. Further, the recent portable equipment is designed to suppress the power consumption as much as possible by allowing an unnecessary circuit block to stand by. If a nonvolatile memory serving as a high-speed work memory and a large-capacity storage memory can be realized, wasted power consumption and memory can be eliminated. In addition, the “instant-on” function that enables instantaneous activation when the power is turned on can be exerted if the high-speed and large-capacity storage nonvolatile memory can be realized.
As the nonvolatile memory, a flash memory using a semiconductor material and a ferroelectric random access memory (FERAM) using a ferroelectric material, and the like may be cited. However, the flash memory has a disadvantage with a slow writing speed in the order of microseconds. On the other hand, in the FERAM, the rewritable times is 1012 to 1014. The problems that the rewritable times of the FERAM are not sufficient for replacement of an SRAM or DRAM with the FERAM, and microfabrication of the ferroelectric layer is difficult are pointed out.
As a nonvolatile memory that does not have these disadvantages, a nonvolatile magnetic memory element called an MRAM (magnetic random access memory) attracts attention. Among the MRAMs, an MRAM using a TMR (tunnel magnetoresistance) effect attracts a lot of attention because of the recent improvement in characteristics of the TMR material. The TMR-type MRAM has a simple structure and is easy to be scaled, and has many rewritable times because recording is performed by the rotation of magnetic moment. Furthermore, a very high speed is expected with respect to the access time, and it is said that the MRAM has already been operable at 100 MHz.
Now, in the MRAM, in order to stably hold the recorded information, it is necessary that the recording layer for recording information has a certain coercive force. On the other hand, in order to rewrite the recorded information, a certain degree of current should be flown in the bit-line. However, with the miniaturization of the MRAM, the bit-lines become thinner, and it is becoming difficult to flow a sufficient current. Accordingly, as a configuration capable of magnetization reversal with a smaller current, a spin injection magnetoresistance-effect element using magnetization reversal by spin injection attracts attention (e.g., see JP-A-2003-17782). Here, the magnetization reversal by spin injection is a phenomenon that electrons spin-polarized through a magnetic material are injected into another magnetic material, and thereby, magnetization reversal occurs in the other magnetic material. In the spin injection magnetoresistance-effect element, compared to the MRAM, the device structure can be made simpler. Further, since the magnetization reversal by spin injection is utilized, compared to the MRAM in which magnetization reversal is performed by an external magnetic field, the element has advantages that the writing current is not increased even when the miniaturization of the element is advanced and that the cell area can be reduced. However, with miniaturization, deterioration in data retention characteristic due to thermal disturbance becomes problematic.
In an in-plane magnetization-type spin injection magnetoresistance-effect element in related art, shape magnetic anisotropy of the recording layer is utilized for recording and holding data. Further, in order to solve the problem of deterioration in data retention characteristic due to thermal disturbance or the like, the ratio of a length along the axis of easy magnetization to a length of the axis of hard magnetization (aspect ratio) of the recording layer is largely taken. Hence, in the solution, it is difficult to further reduce the cell size.